A wireless power transmission or a wireless energy transfer refers to a technology of wirelessly transferring electric energy to desired devices. In the 1800's, an electric motor or a transformer employing the principle of electromagnetic induction has been extensively used and then a method for transmitting electrical energy by irradiating electromagnetic waves, such as radio waves or lasers, has been suggested. Actually, electrical toothbrushes or electrical razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction. Until now, the long-distance transmission using the magnetic induction, the resonance and the short-wavelength radio frequency has been used as the wireless energy transfer scheme.
In the case of a short-distance wireless power transmission, which has been spotlighted in these days, a wireless power transmitter is installed in a building in such a manner that a mobile device, such as a cellular phone or a notebook computer, can be continuously charged when a user uses the mobile device in the building even if the mobile device is not connected to an additional power cable.
In the above wireless power transmission technologies, a coupling coefficient between a wireless power transmitter and a wireless power receiver must be equal to or higher than the critical value in order to effectively perform the wireless power transmission using resonance. At this time, the coupling coefficient may be determined depending on the size of a transmission resonant coil of the transmitter and a receiving resonant coil of the receiver and the distance between the transmitter and the receiver.
In general, the size of the receiving resonant coil is significantly smaller than the size of the transmission resonant coil, so the coupling coefficient between the transmission resonant coil and the receiving resonant coil is very small. Accordingly, energy transmission efficiency may be reduced due to the resonance between the transmission apparatus and the receiving apparatus. Therefore, a repeater resonant unit is installed between the wireless power transmitter and the wireless power receiver to relay the wireless power through the repeater resonant unit.
However, according to the wireless power transmission of the related art, power must be constantly transmitted or relayed regardless of the existence and location of the wireless power receiver that receives the power, causing the waste of the power and damage to the human body